This invention relates to a battery and a method of fabricating thereof. More particularly, it relates to a battery which secures safety by suppressing a temperature rise due to a short-circuit, etc. and a method of fabricating thereof.
In recent years, with the development of electronic equipment, batteries used therein as a power source have increasingly gained in capacity and output density. A lithium ion secondary battery is attracting attention as a battery fulfilling these requirements. A lithium ion secondary battery has an advantage of high energy density but requires sufficient measures for safety because of use of a nonaqueous electrolytic solution.
Conventionally proposed safety measures include a safety valve which relieves an increased inner pressure and a PTC element which increases resistivity on heat generation due to an external short-circuit to shut off the electric current. For example, incorporation of a safety valve and a PTC element into the cap of a positive electrode of a cylindrical battery is known as disclosed in JP-A-4-328278. However, on the safety valve""s working, moisture in the air enters the inside of the battery, which can induce an exothermic reaction in case lithium exists in the negative electrode.
On the other hand, a PTC element, which cuts of f the external circuit involving a short-circuit, exerts no bad influence on operating. The PTC element can be designed to operate when the battery temperature rises to, for example, 90xc2x0 C. or higher due to an external short-circuit so as to be the first safety element to operate in case of abnormality.
Having the above-mentioned constitution, conventional lithium secondary batteries have the following problems.
When a short-circuit occurs in the inside of a conventional lithium secondary battery to raise temperature, the battery is incapable of suppressing an increase of the short-circuit current.
When a short-circuit occurs in the inside of a lithium secondary battery to raise temperature, a separator made of polyethylene or polypropylene interposed between a positive electrode and a negative electrode is expected to soften or melt to clog the pores of the separator, whereby the separator would exude the nonaqueous electrolytic solution contained therein or seal the nonaqueous electrolytic solution within itself to reduce its ion conductivity thereby to diminish the short-circuit current. However, the part of the separator distant from the heat generating part does not always melt. Besides, in case temperature rises, it is likely that the separator melts and flows to lose its function of electric insulation between positive and negative electrodes, which can lead to a short-circuit.
In particular, in the case of a lithium ion secondary battery, the negative electrode is prepared by coating a substrate functioning as a current collector, such as copper foil, with a slurry comprising a negative electrode active material such as graphite, a binder such as polyvinylidene fluoride (PVDF), and a solvent, and drying the coating layer to form a film. The positive electrode is similarly prepared in a film format on a substrate functioning as a current collector, such as aluminum foil. The positive electrode contains a positive electrode active material, such as LiNiO2, a binder, and a conducting agent.
The conducting agent is to enhance electron conductivity of the positive electrode in case where the active material has poor electron conductivity. The conducting agent to be used includes carbon black (e.g., acetylene black) and graphite (e.g., artificial graphite KS-6, produced by Lonza).
When the temperature of such a battery increases to or above the temperature at which the separator melts and flows due to, e.g., an internal short-circuit, a large short-circuit current flows between the positive and negative electrodes at the part where the separator flows as mentioned above. It follows that the battery temperature further increases by heat generation, which can result in a further increase of the short-circuit current.
In case where LiNiO2 is used as a positive electrode active material, there arises a problem that the rate of a temperature rise due to heat generation is higher because it has a greater energy density and causes a higher current value in case of a short-circuit as compared with LiCoO2 which is now in wide use.
The invention has been made in order to solve the above-described problems. An object of the invention is to provide a highly safe battery which is constructed by using an electrode whose resistivity increases with temperature so that an increase in short-circuit current may be suppressed in case the battery temperature should rise due to heat generation by a short-circuit, etc.
A first battery according to the invention is a battery in which a positive electrode has an active material containing nickel, at least one of the positive electrode and a negative electrode has an active material layer comprising an active material and an electron conductive material in contact with the active material, and an electrolyte layer is interposed between the positive and the negative electrodes, which is characterized in that the electron conductive material contains a conductive filler and a resin and is constituted so as to increase its resistivity with a rise in temperature. According to this aspect, since the electron conductive material contains a conductive filler and a resin and is constituted so as to increase its resistivity with temperature, an increase of electric current flowing through the electrode can be suppressed when temperature rises due to heat generation by a short-circuit, etc. There is thus provided a highly safe battery.
A second battery according to the invention is the above-described first battery in which the resin comprises a crystalline resin. The resin comprising a crystalline resin according to this aspect, the rate of increase in resistivity (i.e., the rate of change in resistivity) with temperature can be heightened so that there is provided a battery which can quickly suppress an increase in current flowing through the electrode in case of a temperature rise.
A third battery according to the invention is the above-described first battery in which the resin has a melting point ranging from 90xc2x0 C. to 160xc2x0 C. According to this aspect, since a resin having a melting point of 90 to 160xc2x0 C. is used, the electron conductive material shows an increased rate of resistivity change at around a predetermined temperature within a range of from 90 to 160xc2x0 C. thereby achieving security consistent with battery characteristics.
A fourth battery according to the invention is the above-described first battery in which the electron conductive material is present in an amount of 0.5 to 15 parts by weight per 100 parts by weight of the active material. With the electron conductive material content ranging from 0.5 to 15 parts by weight per 100 parts by weight of the active material, the electrode has a reduced resistivity before the rate of resistivity change with temperature increases. As a result, the battery can have an increased discharge capacity.
A fifth battery according to the invention is the above-described first battery in which the proportion of the conductive filler in the electron conductive material is 40 to 70 parts by weight. The conductive filler content in the electron conductive material ranging from 40 to 70 parts by weight, the electrode shows an increased rate of change in resistivity in case of a temperature rise while having a reduced resistivity in its normal state, and the battery has an increased discharge capacity.
A sixth battery according to the invention is the above-described first battery in which the electron conductive material has a particle size of 0.05 to 100 xcexcm The particle size of the electron conductive material ranging from 0.05 to 100 xcexcm, the electrode has a reduced resistivity before the rate of resistivity change with temperature is increased, and the battery can have an increased discharge capacity.
A seventh battery according to the invention is the above-described first battery in which the conductive filler is a carbon material or a conductive non-oxide. Containing a carbon material or a conductive non-oxide as a conductive filler, the electrode has enhanced conductivity.
An eighth battery according to the invention is the above-described first battery in which the positive electrode contains a conducting agent. Since the positive electrode contains a conducting agent, the resistivity of the electrode can be properly adjusted even in using an electron conductive material having low electron conductivity.
A first method of fabricating a battery according to the invention comprises the steps of:
(a) pulverizing an electron conductive material containing a conductive filler and a resin to prepare fine particles of the electron conductive material,
(b) dispersing the fine particles of the electron conductive material and an active material containing nickel in a dispersing medium to prepare an active material paste,
(c) pressing the active material paste having being dried at a prescribed temperature (T1) under a prescribed pressure to form a positive electrode, and
(d) laying the positive electrode, an electrolyte layer, and a negative electrode one on top of another.
Comprising the steps (a) to (d), the process provides a battery which can suppress an increase in current flowing through the electrode. Further, having the step (c), the process secures good adhesion between the electron conductive material and the active material so that the resistivity of the electrode prepared can be reduced.
A second method of fabricating a battery according to the invention is the first process wherein the resin comprises a crystalline resin. The resin comprising a crystalline resin according to this aspect, the rate of increase in resistivity (i.e., the rate of change in resistivity) with temperature can be heightened so that there is provided a battery which can quickly suppress an increase in current flowing through the electrode in case of a temperature rise.
A third method of fabricating a battery according to the invention is the first process in which the prescribed temperature (T1) is the melting point of the resin or thereabouts. By setting the prescribed temperature (T1) at or around the melting point of the resin, the improvement in adhesion between the electron conductive material and the active material is further ensured so that the resistivity of the electrode prepared can be reduced further.